The present invention is in a process of producing phosphate coatings on surfaces which consist of aluminum or its alloys and of at least one of the materials steel and galvanized steel, wherein said surfaces are treated by spraying or by spraying/dipping with a phosphatizing solution which contains zinc ions, phosphate ions, fluoride ions and an accelerator and the use of the process in preparing the surfaces for an electrophoretic dip painting.
Metal surfaces which consist of aluminum and its alloys and of steel and/or galvanized steel can be phosphatized by being sprayed with zinc phosphate solutions which contain fluoride. Such a phosphatizing treatment has proved satisfactory in the normal-zinc processes in which the phosphatizing solution has a zinc content of, e.g., 2 to 6 g/l.
However, there are considerable difficulties in the use of that technology in the low-zinc spray-phosphatizing processes using a phosphatizing solution which has a zinc content below 1 g/l. One serious problem is that the uniformity and the cover factor of the phosphate coatings formed on the aluminum exhibit considerable variation so that processing cannot be satisfactorily used in practice.
It is an object of the invention to provide a process which is free of the disadvantages mentioned above for the production of phosphate coatings on surfaces consisting of aluminum or its alloys and at least one of the materials steel and galvanized steel and which particularly results in uniform phosphate coatings and can be carried out without a particularly high expenditure regarding the components of the solution and the performance of the process.